We briefly discuss a statistical mechanical theory from which voltammogram spikes observed during underpotential deposition (UPD) on electrode surfaces can be obtained. We interpret a voltammogram spike as an averaged result of a first-order phase transition taking place in each of the crystalline domains (crystals) formed on the electrode surface. This allows us to fit the theoretical voltammogram spike to experiment with very good precision. We demonstrate this with the UPD of Cu on Pt(111).